This invention relates in general to the field of drying systems, and in particular to a method and apparatus for acoustic and vibrational energy assisted cleaning and drying of electronic modules and solder stencils.
Hot air drying systems are an established method of drying bare Printed Circuit Boards (PCB""s), various components on a (PCB), and tooling which may require cleaning such as stencils, board supports, and the like. There are a wide variety of equipment and processes available to manufacture, solder, clean and dry PCBs, and to clean and dry stencils and other tooling, however, the general principles of the process remain the same, as explained below.
After components have been assembled to the PCB, the assembly is often referred to as a module. Flux residue and other contaminants may remain on the module after assembly, necessitating the cleaning and drying process. These residues may be similar to those found on stencils and other tooling required for the assembly process. The module, stencil or other tooling may be cleaned in an aqueous cleaning system used to remove flux residue or other contaminates such as solder balls associated with the component or module manufacturing process. Once cleaned, it is important to remove all of the moisture from the interior of open components on the module and the exterior of the module. It is also necessary to remove moisture from stencils or other tooling required in the process.
The limitations of this process continue to be challenged with the inclusion of smaller openings within connectors, smaller gaps under components, and the like which can entrap moisture. Any excess water or moisture will cause corrosion over time. This is especially a problem when power is applied to a module which is not dry, causing a galvanic reaction and, therefore, corrosion.
Tooling, such as solder stencils and wave solder pallets, require cleaning after becoming contaminated with either solder paste or flux residue. Solder stencils need to have any remaining solder paste removed prior to storage. If the solder paste dries within the apertures of the stencil, the dried solder paste will interfere with the release of the solder paste during the next assembly process and cause defects. Build up of flux residue on wave solder pallets will hinder the application of the flux onto the assembly and cause defects.
The solder stencil printing process sometimes includes an under wiping process. The under wiping process may further apply a solvent to either the under wiping paper or the underside of the stencil. The under wiping process can further include a vacuum system which removes the loose solder particles located inside the apertures of the stencil and any solvent in the direct flow of the vacuum. The under wiping process may not sufficiently remove excess solvent remaining on the top-side of the stencil. The process may not remove all of the solder residue within the apertures of the stencil.
After completion of the reflow and/or wave soldering processes, the assemblies are cleaned to remove the remaining residue or contaminants. The cleaning process applies some form of liquid, generally water. Chemicals with relatively low flash points were used in the past, but those chemicals are expensive and some were found to be harmful to the environment. One of the more desirable chemicals used to clean assemblies is water. Water, or other cleaning solutions with similar flash points, are difficult to dry in a short time period. The desirable outcome of the drying process is for components and the module to be sufficiently dried to preclude corrosion. Various processes and devices are available to dry electronic modules.
In one case, hot air is blown over and across the module with sufficient velocity, volume and thermal content to evaporate some of the moisture and urge some of the remaining moisture off the module. The limitations of this are that the dryers require a great deal of thermal energy and large capacity air blowers to provide sufficient drying. Additionally, these dryers are generally loud and require sound dampening. Drying depends on convection of hot gases past the module. The rate of drying decreases after a portion of moisture has been removed. The last few points of moisture removal take the longest and increase the cost of drying. If one attempts to increase the temperature of the drying gas, there is a risk of thermally damaging the electronic components on the module. The efficiency of drying is proportional to the temperature of the drying gas. Thermally damaging the module sets a practical upper limit for the gas temperature. Additionally, this process continues to be limited when moisture is trapped in components such as connectors.
In another case, infrared energy is applied to the module in an attempt to evaporate excess moisture. This process is somewhat limited by the time required for drying excess moisture. Because of this limitation, infrared dryers are often used in conjunction with hot air dryers. Infrared energy transfers heat to the exposed surfaces; where the infrared energy would have a difficult time to evaporate entrapped moisture from within pockets of components such as connectors or under components designed to have a space between them and the surface of the PCB, such as ball grid array packages.
In another case, reference is made to U.S. Pat. No. 4,334,366 which teaches a method of drying objects in a perforated drum. Hot gas and sonic energy are used to dry the food objects which are tumbled in the perforated drum, and upon sufficient drying, the objects are removed from the drum. The limitations of this patent are that electronic modules cannot be tumbled in a drum and are most often processed on a conveyor to preclude damage to the module.
In yet another case, reference is made to U.S. Pat. No. 3,592,395 filed Sep. 16, 1968, to Lockwood et. al. This dryer uses pulsating hot gas and sonic energy to dry a stirred slurry. This dryer readily handles slurries or other fine powdery materials. This type of dryer would not work with electronic modules as any stirring of electronic modules would cause mechanical damage to the modules.
In yet another case, reference is made to U.S. Pat. No. 5,113,882 filed Aug. 28, 1990 to Gileta. A dryer system for a liquid cleaning apparatus has a dehumidifier to remove vapors, droplets of liquid cleaning agent and recirculate dry gas onto workpieces moving on a conveyor. Gileta teaches lowering the relative humidity within the atmosphere to increase the efficiency of the drying of printed circuit assemblies.
Ultrasonic transducers are used in wave soldering technology to atomize liquid flux into a fine mist and transferring the flux in mist form from the source reservoir to the bottom side and into the plated through holes of the module. This is commonly referred to as a spray fluxer. This clearly shows that moisture can be atomized when near or contacting a source of vibrational energy.
It can be recognized that improvements made to the drying process of modules, can also be utilized in the drying processes applied to tooling as well as stencils within solder printers and stencils in stencil cleaners.
While each of these improvements has contributed to the art, the limitations of these processes continue to be challenged.
Thus, what is desirable, is a means to reliably clean and dry electronic modules and tooling utilizing a minimal amount of energy and time and precluding any mechanical or thermal damage to the module.
The present invention addresses the deficiencies in the art by applying acoustic pressure waves and vibrational energy proximate to the surface of the module or tooling such that the energy aids in the improvement of drying of electronic modules. The contacting and/or non-contacting pulsating energy increases the drying efficiency over the prior art solutions by atomizing moisture droplets into a fine mist, as well as allowing the combined use of prior art solutions such as hot air blowers and infrared energy. The deficiencies within the present art may further be addressed by including the same vibrational energy in conjunction with the cleaning fluid during the cleaning process to further aid in cleaning the object.
One aspect of the current invention is to provide a means to apply acoustic pressure waves to the desirable side of a module causing the atomizing of excess moisture.
A second aspect of the present invention is the use of ultrasonic sources for the acoustic pressure waves.
A third aspect of the present invention is to provide a means to use acoustic pressure waves to atomize moisture in small openings, crevices and hard to reach places.
A fourth, aspect of the invention is to combine at least one of acoustic pressure waves and vibrational energy to prior art solutions such as hot air knives allowing improved efficiency and lower time and energy costs.
A fifth aspect of the present invention is the ability to dry tooling such as wave solder pallets or solder stencils.
A sixth aspect of the present invention is the inclusion of a vibrational energy source within an under stencil cleaner within an automated or semi-automated solder paste printing apparatus.
A seventh aspect of the present invention is the inclusion of a vibrational energy source in conjunction with a fluid to aid in the cleaning process.
An eighth aspect of the present invention is the inclusion of a vibrational energy source in conjunction with the cleaning process of an under stencil wiper.
A ninth aspect of the present invention is the inclusion of a vibrational energy source to atomize residual moisture on stencils.
A tenth aspect of the present invention is the inclusion of a vibrational energy source to atomize residual moisture on stencils, used in conjunction with an under stencil cleaner.
An eleventh aspect of the present invention is the combination of using the vibrational energy source for both cleaning and drying.
A twelfth aspect of the present invention is the inclusion of a vibrational transfer medium placed between the vibrational energy source and the object.
A thirteenth aspect of the present invention is the use of air to transfer the vibrational energy to the object.
A fourteenth aspect of the present invention is the use of at least one of ultrasonic transducer, speaker, tuning fork, horn, and sonar transducer to generate the vibrational energy.
A fifteenth aspect of the present invention is the application of the disclosed technology within an automated in-line cleaner.
A sixteenth aspect of the present invention is the application of the disclosed technology within an automated stencil cleaner.
A seventeenth aspect of the present invention is the application of the disclosed technology within an automated solder printer.
An eighteenth aspect of the present invention is a means for changing the relational proximity of the vibrational energy source and the at least one of electronic assemblies and tooling related to manufacture of electronic assemblies to one another. One example would be a conveyor. A second would be a means for moving the vibrational energy source.
The present invention comprises an apparatus used to apply acoustic pressure waves and resulting vibrational energy to a module to atomize moisture during the drying phase of assembly of the module. The apparatus preferably includes equipment known in the art: a conveyor to move the module through an in-line cleaner wash, rinse, and drying sections, which can include hot air blowers and infrared heaters. Alternatively, the present invention may be included within other equipment known in the art such as stencil cleaners and batch cleaners or solder printers.
The present invention discloses the application of acoustic pressure waves and resulting vibrational energy to atomize excess moisture and entrapped moisture in hard to reach cracks and crevices, thus overcoming the surface tensional forces and allowing increased efficiency of the hot air dryers and the infrared heaters. The acoustic pressure waves, generated by a transducer and transferred to the module through the air, will impinge the module at the angle proscribed and not be substantially affected by the volume or velocity of the hot air flow caused by the hot air dryers.
The present inventions further discloses the application of vibrational energy through close proximity to dry preferably planar tooling such as stencils where heat is not desirable.